In vehicle loading cranes it is important, from the point of view of work safety, that during loading the vehicle is not exposed to such a strong moment of lifting that the vehicle runs the risk of tipping over.
The present invention more particularly relates to an arrangement in hydraulically operated cranes of the type provided with a loading arm which is mounted on the crane post for swinging movement in a vertical plane relative to said post and is provided with a number of hydraulic piston-and-cylinder units by means of which it is possible to regulate the distance of the load supported in the crane arm to the crane supporting post.
The arrangement in accordance with the invention is particularly intended for use in hydraulic loading cranes of the type being provided with a multiple loading arm, e.g. consisting of one inner arm portion and one outer arm protion, which may be raised and lowered in a vertical plane, and with an extension arm, a so called projection arm, which may be displaced in the longitudinal direction of the loading arm. The hydraulic system of the crane comprises valves to operate the hydraulic piston-and-cylinder units effecting the movements of the two loading arm portions and the extension arm, respectively, and a valve controlled by the liquid pressure in the loading arm hydraulic cylinder, called the master cylinder. hitherto, this valve has been arranged, upon excess of a predetermined maximum liquid pressure in the master cylinder, to actuate the means operating the valves contrlling the liquid supply to and the liquid discharge from the master cylinder and the hydraulic cylinder for the extension of projection arm. A device of this kind is rather complicated and bulky and in addition, it has often proved not completely reliable in operation.